Robust heterojunction to strengthen the performances of FAPbI3 perovskite solar cells

• Formation of (TMA) x FA 1-x Pb(PF 6 ) x I 3-x /FAPbI 3 heterojunction through interface ion exchange. • Strong hydrogen bonding and ion size effect stabilize the phase structure. • A champion efficiency of 21.43% was obtained with greatly enhanced device stability. Though formamidinium lead triiodide FAPbI 3 is hotly researched, it still suffers from chemical and phase instabilities, arising from inherent weak chemical bonding and easily transformed crystal configures respectively. This induces depraved opto-electronic properties and device instability, which is imperative to be solved. Here, chemical interaction enhancement in heterostructure of (TMA) x FA 1-x Pb(PF 6 ) x I 3-x /FAPbI 3 simultaneously improves the chemical and phase stability with preserving its inherent bandgap. Strong hydrogen bonding between PF 6 - and FA + drastically reduces the α phase formation energy and diminishes the interface trap states. The resulting perovskite solar cells achieve improved efficiency of 21.43% compared with 19.32% of the control device. Meanwhile, ∼90% and 80% of the initial efficiency are maintained for unpacked devices after 1200 h storage (25 °C, 30% relative humidity) and 300 h maximum power point tracking (1 sun, 25 °C, N 2 ), outperforming the control one. This work highlights a robust interface construction method to overcome the inherent problem of FAPbI 3 .